1. Field of the Invention
This invention relates to medical imaging systems using nuclear magnetic resonance. In a primary application the invention relates to projection imaging over selected regions.
2. Description of Prior Art
Nuclear magnetic resonance, abbreviated NMR, represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, magnetic moments are excited at specific spin frequencies which are proportional to the local magnetic field. The radio frequency signals resulting from the decay of these spins are received using pick-up coils. By manipulating the magnetic fields, an array of signals are provided representing different regions of the volume. These are combined to produce a volumetric image of the density of the body.
A descriptive series of papers of NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR" by W. V. House, pp. 1220-1226.
A number of three-dimensional methods are described. One important one is described by P. V. Lauterbur and C. M. Lou entitled, "Zeugmatography by Reconstruction from Projections," pp. 1227-1231. In this approach, a linear field gradient is superimposed on the strong axial magnetic field. As a result of the gradient, each plane in the volume, in a direction normal to the gradient, experiences a different resonant frequency. A burst, containing a spectrum of frequencies, is used to simultaneously excite each of the planes. The received signal, following the excitation, is then Fourier transformed into its individual components. The amplitude at each frequency represents a planar integration of the proton density. This process can be repeated using a gradient field in different directions to collect information about arrays of planes. These planar integrals can be used to produce two-dimensional projection images of a volume or, alternatively, three-dimensional information about the proton density of each voxel in the volume.
The projection image is obtained by obtaining the integrated density of substantially all planes which are normal to the plane of the projection image. The total number of planes required, at all angles and positions, is substantially equal to the number of pixels in the two-dimensional projection image. The reconstruction procedure involves the classical reconstruction from projections widely used in current computerized tomography systems. The most generally used procedure is that of convolution-back projection.
The resultant two-dimensional projection images have a number of drawbacks and, as a result, are not used. Firstly, the superimposed intervening structures make it very difficult to visualize the desired structure, be it an organ or tumor. Secondly, the nature of this imaging procedure is such that all of the measurements affect every reconstructed pixel. This makes the image particularly sensitive to motion. Any motion of the object will cause artifacts in the image due to inconsistencies where the object does not match its projections. These artifacts can often obscure the desired information.
To avoid the problems of intervening structures, three-dimensional reconstructions are made which provides cross-sectional images. The approach taken in the Lauterbur paper involves making an array of two-dimensional projection images at every angle through the object. Lines in these projection images represent line integrals or projections of cross-sectional planes of the object. Thus, again using classical reconstruction techniques, any desired cross-sectional plane can be reconstructed. The intermediate two-dimensional projections are not used for the reasons discussed.
Although these cross-sectional images are free of intervening structures, they are unsuitable for many medical problems. The cross-sectional format is often difficult to interpret especially in the imaging of blood vessels. In addition, the acquisition of three-dimensional data takes a relatively long time, thus resulting in a variety of artifacts due to the various physiological motions of the body.
A number of articles and books have been published on the wide variety of ways of accomplishing cross-sectional or three-dimensional imaging using NMR. These include books Nuclear Magnetic Resonance Imaging in Medicine, published in 1981 by Igaku-Shoin Ltd., Tokyo, NMR Imaging in Biomedicine, by P. Mansfield and P. G. Morris published in 1982 by Academic Press, and review papers "NMR Imaging Techniques and Applications: A Review," by Paul A Bottomley in the Review of Scientific Instruments, Vol. 53, pp. 1319-1337, 1982, and "Fourier Transform Nuclear Magnetic Resonance Tomographic Imaging," by Z. H. Cho, et al., Proc. of the IEEE, Vol. 70, pp. 1152-1173, 1982. In each of these, as previously indicated, systems are described of producing cross-sectional or three-dimensional images. These are unsuitable in many clinical studies where it is important to view an organ in its entirety in a volume. This is particularly true in the vital application of vessel imaging where, as in existing angiography, vessel narrowings are studied by viewing a projection image of a vessel in a volume. A cross-sectional image showing slices through vessels has limited diagnostic value.
The subject of projection imaging in NMR was first introduced by the applicant in two U.S. patent application Ser. No. 332,925 "Blood Vessel Imaging System Using NMR" and Ser. No. 332,926 "Selective Material Projection Imaging System Using NMR." These were followed by a publication in the IEEE Transactions on Medical Imaging, Vol. 1, No. 1, pp. 42-47, 1982 entitled "Selective Projection Imaging: Applications to Radiography and NMR." In addition, the applicant has submitted two additional pending applications on this same subject matter entitled "Improved Blood Vessel Projection Imaging System Using NMR," and "Improved Selective Material Projection Imaging System Using NMR."
Each of these patent applications and the publications address a vital problem in projection imaging, namely the selective imaging of a specific region within the body. In X-ray we are limited to projecting the entire volume, since each X-ray photon must enter on one side of the volume of interest and emerge on the other. In NMR, however, we can accomplish the very important function of projecting over a desired portion of the volume. This can be vital in applications such as the coronary arteries where many intervening structures such as pulmonary vessels and heart chambers can obscure the desired image. Thus, in projection imaging, region selectivity can be vital.
In the previous applications a number of methods were shown, including the insensitization of the undesired regions of the volume so that only the desired regions will project. An improved version of this method involved further insensitization preceding each data acquisition. However, what is most desired, is a system of accurately exciting only the region of interest so that undesired regions are automatically excluded. This would take care of those circumstances where the previous methods might prove inadequate or overly complex.